Virtually all organic substances are electrically insulative. In recent years, however, certain groups of organic polymers possessing electroconductivity and known as organic semiconductors have been attracting growing attention. Generally, the organic substances which are electroconductive in themselves are classified into three types.
The first type of such electroconductive substance is graphite. Strictly speaking, graphite is not regarded as an organic substance. However, graphite may be considered to possess a structure where an organic conjugated system extends to a high degree. Graphite in its unmodified form already possesses fairly high electroconductivity. By intercalation of another compound, it is possible to provide enhanced electroconductivity, possibly even reaching the level of superconductivity. Graphite, however, is strongly two-dimensional, and is not easily fabricated into a desired shape. This fact prevents graphite from finding extensive utility in many applications.
The second type is charge-transfer complexes. The crystalline substance obtained by using, for example, tetrathifulvalene and tetracyanoquinodimethane, as an electron donor and an electron acceptor, respectively, possesses extremely high electroconductivity of 400 to 500 S/cm at room temperature. Since these charge-transfer complexes are not polymers, they are deficient in moldability, and, similarly to graphite, can not find utility in many practical applications.
The third type is .pi. electron conjugate type organic polymers, represented by polyacetylene, which can acquire high electroconductivity by doping. Before doping, polyacetylene of the trans form exhibits electroconductivity of 10.sup.-5 S/cm, and polyacetylene of the cis form exhibits elctroconductivity of 10.sup.-9 S/cm; i.e., they possess properties approximating the properties of semiconductors or insulators. When these polyacetylenes are doped with an electron acceptor such as, for example, arsenic pentafluoride, iodine, sulfur trioxide, or ferric chloride or with an electron donor such as an alkali metal, they can form p-type semiconductors and n-type semiconductors, respectively. They can further acquire electroconductivity as high as 10.sup.3 S/cm. The above-described polyacetylenes are electroconductive organic polymers, interesting from the theoretical point of view. They are, however, highly susceptible to oxidation, so that, if left standing in the atmosphere, they are readily degraded by oxidation and seriously altered in quality. In the doped state, the polyacetylenes are more sensitive to oxidation, and decrease their electroconductivity sharply upon exposure to even a low degree of moisture in the air. This trend is particularly remarkable in the n-type semiconductors of polyacetylenes.
Poly(p-phenylene) and poly(p-phenylene sulfide), before doping, possess low degrees of electroconductivity, viz., 10.sup.-9 S/cm and 10.sup.-16 S/cm, respectively. When they are doped with, for example, arsenic pentafluoride, they are converted into electroconductive organic polymers possessing electroconductivities of 500 S/cm and 1 S/cm, respectively. The electric properties of such doped organic polymers are unstable, though, to varying degrees.
As described above, these doped electroconductive organic polymers generally possess extremely unstable electric properties under atmospheric exposure. This is a phenomenon common to electroconductive organic polymers of this class. This phenomenon has been a hindrance to the growth of use of such polymers in practical applications.
Various electroconductive organic substances have been known in the art as described above. With a view to ensuring growth of their utility in practical applications, such substances should desirably be provided in forms which are excellent in moldability.
On the other hand, research on the oxidative polymerization of aniline as an oxidation dye has long been under way regarding aniline black. Particularly, as an intermediate in the production of aniline black, the octamer of aniline represented by Formula (I) has been identified as emeraldine (A. G. Green et al., J. Chem. Soc., Vol. 97, p. 2388 (1910); ibid, Vol. 101, p. 1117 (1912)). This octamer is soluble in 80% acetic acid, cold pyridine, and N,N-di-methylformamide. ##STR2## The emeraldine is oxidized in an ammoniacal medium to form nigraniline represented by Formula (II), which is also known to possess solubility similar to that of emeraldine. ##STR3##
It has been demonstrated recently by R. Buvet et al. that the sulfate of emeraldine possesses high electroconductivity (J. Polymer Sci., C, Vol. 16, pp. 2931, 2943 (1967); ibid, Vol. 22, p. 1187 (1969)).
It has been also demonstrated that an organic substance similar to emeraldine can be obtained by electrolytic oxidative polymerization of aniline (D. M. Mohilner et al., J. Amer. Chem. Soc., Vol. 84, p. 3618 (1962)). According to this publication, a substance soluble in 80% acetic acid, in pyridine, and in N,N-dimethylformamide can be obtained when an aqueous sulfuric acid solution of aniline is subjected to electrolytic oxidative polymerization using a platinum electrode at an oxidation potential of +0.8 V relative to the standard calomel electrode, a level necessary for avoiding electrolysis of water.
In addition to the reports mentioned above, Diaz et al. (J. Electroanal. Chem., Vol. 141, p. 141 (1980)) and Oyama et al. (Polymer Preprints, Japan, Vol. 30, No. 7, p. 1524 (1981); J. Electroanal. Chem., Vol. 161, p. 399 (1984)) have also tried electrolytic oxidative polymerization of aniline. All these studies are aimed at polymer-coated chemically modified electrodes where the electrolysis is conducted at potentials not exceeding 1 V.